Such actuators are deployed in particular in motor vehicles. They are used there for example as actuators for sliding roofs, window lift systems, door locking systems, windshield wipers, solar protection systems and other power-activated control elements. The drive units deployed in such cases are preferably permanent-field direct current electric motors. Electronic control units linked to corresponding sensors are deployed to activate the drive units, monitor the current position of the actuators and to identify or monitor further parameters, e.g. actuating force and actuating speed.
Such an actuator with an electric motor is for example known from the utility model DE 295 14 786 U1. An actuator, in particular in the form of an electric motor transmission unit, is disclosed there for moving power-activated windows, partition walls or roof element systems in motor vehicles with a defined closing force limit in the event of impact with an obstacle. For this the actuator is controlled via a control device as a function of detected stroke position, so that the maximum permissible closing force is not exceeded in the event of impact with an obstacle. Stroke position detection is thereby a function of a speed or position sensor of the actuator, for example a speed or position sensor assigned to the rotor shaft of the electric motor. The respective stroke position of the window is detected from the rotor position of the commutator motor based on a first recorded initialization position and subsequent detection of the rotation of the rotor shaft via the speed/rotation angle or position sensor in the control device.
The patent document DE 196 38 781 C2 also discloses such an actuator, in particular for moving windows, partition walls or roof element systems in motor vehicles. Actuators of this type contain a commutator motor, the rotor shaft of which engages as a transmission shaft with a worm wheel, which drives the respective closing element. To achieve effective protection against trapping with low outlay with regard to control and measurement technology, a speed-dependent travel sensor is used, to determine the speed limit value of significance for closing force limitation in the control unit, taking into account sluggishness as a function of the stroke.
A further actuator for moving a vehicle window is known from the patent document DE 195 14 954 C2. With this actuator the closing force is limited by detecting the motor current and stroke of the window. When adjustable limit values are reached, the drive is disconnected, whereby the limit value is defined so that a specific motor current is stored in a control unit for every window position.
With actuators of the above type it is necessary for the control unit to know the current position of the control element all the time during operation. As changes in the position of the control element, as a function of the transmission mechanism, can also result due to the action of an external force on the control element, the sensor arrangement for detecting the position of the control element must be kept in permanent standby mode, even when the actuator is at rest. This requires a constant supply of power to the electrical sensor elements and associated electronic control system, therefore constant energy consumption. As particularly when using such actuators in motor vehicles, the energy available in the vehicle electrical system is limited, it is particularly important to keep energy consumption to a minimum.